How to set different colors to each side of plane
I think you can do it by making it "physically" double-sided, which means:


In Edit mode, Select all faces, then ShiftDEnter to duplicate it, then CtrlN (or CtrlShiftN) to flip normals for all faces.
Add a new material and assign to the selection.
Make sure to toggle Normals when exporting.




It would work if WebGL can do single-sided render (I bet it can), which is normal-based, like GLSL. Or you can make two separate objects that overlap each other, and have their face normals completetly flipped.



If you just want to render it in Blender, here is the solution for BI:



And for Cycles: